1. Field of the Invention
The invention relates generally to light shaping diffusers, and more particularly, to a surface light shaping diffuser formed from a monolithic glass material and also to a method of forming the surface light shaping diffuser.
2. Description of the Related Art
A Light Shaping Diffuser.TM. (LSD.RTM.), sometimes known as a light shaping homogenizer or simply a diffuser, is a type of diffuser used in a variety of illuminating, imaging, and light projecting applications. A LSD is a transparent or translucent structure having an entrance surface, an exit surface, and light shaping structures formed on its entrance surface and/or in its interior. These light shaping structures are random, disordered, and non-planar microsculpted structures. These structures are created during recording of the medium by illuminating the medium with a speckle pattern produced in conjunction with coherent light or the combination of incoherent light and a computer-generated mask which simulates speckle. The speckle produce changes in the refractive index of the medium which, when developed, are the micro-sculpted structures. These light shaping structures diffract light passing through the LSD so that the beam of light emitted from the LSD's exit surface exhibits a precisely controlled energy distribution along horizontal and vertical axes. LSDs can be used to shape a light beam so that over 90% (and up to 95%-98%) of the light beam entering the LSD is directed towards and into contact with a target located downstream of the LSD. A LSD can be made to collect incoming light and either (1) distribute it over a circular area from a fraction of a degree to over 100.degree., or (2) send it into an almost unlimited range of elliptical angles. For example, a 0.2.degree..times.50.degree. LSD will produce a line when illuminated by a LED or laser and a 35.degree..times.90.degree. LSD will form a narrow field, high resolution rear projection screen when illuminated by the same light source.
Rather than exploiting a property of monochromatic laser light known as coherence that requires that the finished holographic element be used only at the laser's wavelength, a LSD operates perfectly in white light. LSDs therefore exhibit a high degree of versatility because they may be employed with light from almost any source, including LEDs, daylight, a tungsten halogen lamp, or an arc lamp.
Two types of LSDs are currently available, namely a "volume LSD" and a "surface LSD." A volume LSD is a volumetric optical element primarily characterized by the incorporation of light shaping structures within its body and which diffract light passing therethrough. A surface LSD is a surface relief optical element primarily characterized by the incorporation of light shaping structures on its surface and which diffract light passing therethrough. A surface LSD in addition to being produced optically may also be created by mechanical manipulation of the surface of the medium. See below for a list of some pending applications and issued patents related to each. Volume LSDs and surface LSDs are interchangeable in most applications. There are some limited applications, however, in which volume LSDs are preferred, such as applications in which the LSD is submerged in a liquid.
The light shaping structures in volume LSDs are recorded using a coherent light recording system similar to a holographic recording system. Coherent light passed through a master diffuser is incident upon a volumetric photosensitive medium (such as dichromated gelatin DCG or another volume recording material). The speckle pattern in the light incident the medium is rendered in the medium by altering the refractive index of the medium. Where the speckle pattern is bright, the medium is hardened and the refractive index of the medium is increased. Where the speckle pattern is dark, the refractive index remains substantially unchanged. Upon development, these variations in the refractive index are rendered essentially permanent. Alternatively, the speckle pattern may be generated using an incoherent light source and a speckle-imitating mask in a process akin to a printing process. Light passed through the mask is incident upon the volumetric medium and the speckle pattern generates variations in the refractive index of the material essentially as before.
Surface LSDs are produced in similar fashion as well as in alternative ways. Recording set ups similar to those described above are used with the exception that a nonvolume recording medium such as standard photoresist is used in place of a volume medium such as DCG. During development, the areas having increased refractive index due to hardening remain while the softer, lower index areas are washed away. This process leaves microstructures having light shaping properties at the surface of the medium. These structures are then replicated in any number of materials including plastics using various replication techniques such as embossing, injection molding, and epoxy replication.
LSD production is disclosed in U.S. Pat. No. 5,365,354 to Jannson et al. (the '354 patent), U.S. Pat. No. 5,609,939 to Petersen et al. (the '939 patent), and U.S. Pat. No. 5,534,386 to Petersen et al. (the '386 patent). The '354 patent, the '386 patent, and the '939 patent hereby are incorporated by reference for their disclosure of the production of a LSD. Commonly assigned U.S. patent application Ser. No. 08/902,415, to Lieberman entitled "Monolithic Glass Light Shaping Diffuser and Method for its Production" (the '415 application) discloses several methods for fabricating diffusers from a sol-gel glass composition from a plastic or epoxy submaster for high temperature uses. The '415 application is also incorporated herein by reference for its disclosure of LSD production. Other related U.S. patent applications include "Non-Lambertian Glass Diffuser and Method of Making," filed Aug. 20, 1998, "Diffuser Master and Method of Manufacture," filed Aug. 20, 1998, "High Efficiency Monolithic Glass Light Shaping Diffuser and Method of Making," filed Aug. 25, 1998, "Optical Element Having an Integral Surface Diffuser," filed Aug. 25, 1998, "Vehicle Light Assembly Including a Diffuser Surface Structure," filed Aug. 25, 1998, "Apparatus Having a Light Source and a Sol-Gel Monolithic Diffuser," filed Aug. 25, 1998, "Passive Matrix Liquid Crystal Display," filed Aug. 25, 1998, and "Device Including an Optical Element With a Diffuser," filed Aug. 25, 1998. These applications are also incorporated by reference herein.
LSDs heretofore were formed solely from plastics such as acrylic or polycarbonate plastics because only these materials were sufficiently deformable (under conditions suitable for interaction with a submaster) to accept the light shaping structures. Limitations resulting from the physical properties of these plastics restrict the applicable range of LSD operation.
For instance, the plastics from which LSDs are formed typically have a glass transition temperature of below about 150.degree. C. and often below about 100.degree. C. Conventional plastic LSDs therefore cannot be used in applications in which the LSD may be subjected to sufficient heat to raise the temperature of the LSD to above this glass transition temperature. This heat may be received directly from a light source such as an arc lamp or may be absorbed in the form of UV or infrared radiation. Plastic LSDs therefore generally cannot be used in heat lamps, liquid crystal display projectors, projector lamps, track lighting, or other light sources that generate significant heat near the location of the LSD. Plastic LSDs also are not widely usable with light sources operating in the ultraviolet range or infrared range which emit radiation that is absorbed by the plastic.
One limitation of plastic LSDs is that they cannot be subject to a hot coating operation. It is often desirable to coat a diffuser with a layer of an anti-reflective (AR) coating in order to raise the efficiency of the diffuser. Many coatings, including many AR coatings, can be applied only at temperatures above the glass transition temperature of plastics commonly used in LSDs. Conventional LSDs are not usable with these coatings.
Yet another problem associated with a conventional plastic LSD is that it is difficult or impossible to form a high quality three-dimensional lens on its exit surface. It is desirable in a variety of diffuser applications to place a lens on the exit surface of the diffuser. Conventional plastic LSDs cannot be ground, polished, or molded into high quality lenses. High quality lenses can be produced on the exit surface of a LSD only by laminating or otherwise attaching a Fresnel lens on it. As is well known in the art, a Fresnel lens is one having a planar or two-dimensional surface that in use creates an effect that is designed to approximate the effect of a three-dimensional curved lens. Mounting a separate Fresnel lens onto the exit surface of a diffuser is substantially more difficult and expensive than simply grinding or otherwise forming a conventional curved lens on the exit surface and may produce a lower quality lens.
Many of the above-identified disadvantages of a plastic LSD could be avoided if the LSD were to be formed from glass rather than a plastic. However, light shaping structures cannot be embossed on or otherwise recorded in a conventional glass structure during its production process because the high temperatures accompanying formation of conventional glass (on the order of 1,800.degree. C.) would destroy the master or submaster bearing the light shaping structures.
The '415 application noted above discloses a monolithic glass light shaping diffuser construction and a method of making the diffuser from a glass composition known as sol-gel. The '415 application discloses a volume LSD and a method of making the volume diffuser. It also discloses a surface LSD and methods of making the surface diffuser. The surface LSD is formed by a casting process wherein the sol-gel composition is cast in a plastic mold which bears the light shaping structures on an inner surface of the mold. Another method for forming a surface LSD is disclosed in the '415 application whereby a coating or layer of the sol-gel composition forms a film layer on a substrate. A submaster or master diffuser which bears the light shaping relief structures contacts the film layer so that the surface structures are recorded in the film layer after the sol-gel layer undergoes a glass transition, an aging and a heat treating process. The master or submaster which bears the surface relief structures is disclosed as being made from a plastic material.
Depending upon the process utilized to form the sol-gel glass LSD disclosed in the '415 application, the master or submaster from which the surface relief structures are recorded is formed from a substantially rigid and hard plastic material which is very stiff and inflexible. In order for the surface relief structures to be completely and properly recorded into the sol-gel material, the sol-gel material must be sustained in a containing space, coated onto a base substrate or inserted into a mold at a precisely controlled viscosity. If the viscosity varies even slightly less or slightly greater than desired, the sol-gel material may not flow properly and completely contact the surface relief structure of the master. Additionally, the sol-gel material may not flow completely into all of the surface relief spaces if at a slightly undesirable viscosity. The hard plastic material of the master or submaster does not yield, bend or flex at all to aid in having the sol-gel material flow properly. Hence, if the viscosity is not precisely as desired, all of the surface relief structures may not be recorded into the sol-gel material or may be recorded inaccurately.
An additional problem with these present processes is that each time a submaster copy of a particular original master surface relief structure is recorded it loses some of its resolution and therefore provides slightly altered light shaping characteristics. For example, a master photoresist material is typically provided having the surface relief structures recorded therein from which a second generation submaster is created having the features subsequently recorded therein. A third generation submaster is then created from the second submaster having the features subsequently recorded therein as well. Sometimes, other additional submasters are created between the original master and the final diffuser product. Each subsequent formation of the surface relief structures in a subsequently produced submaster creates lower resolution and hence lower quality light shaping characteristics, and thus it would be beneficial to be able to eliminate one of these submaster steps.